A known digital signal receiving apparatus adapted to an FSK (Frequency Shift Keying) system is disclosed in Japanese Patent No. 2001-4736A (hereinafter, referred to as reference 1), for example. FIG. 3 is a block diagram illustrating a general configuration related to a digital signal receiving apparatus disclosed in the reference 1. As illustrated therein, the digital signal receiving apparatus, generally indicated at 70, receives a desired signal S combined with a noise signal N, which is correlated as an ambient noise, by means of a first antenna 71. Then, the digital signal receiving apparatus 70 down-converts the received signals to a desired high-frequency band at a high-frequency processing portion 72. Further then, the digital signal receiving apparatus 70 generates a corresponding digital signal (S″+N″) by converting the signals from analog to digital in an A/D converting portion 73, and outputs the digital signal to a noise-filtering unit 74.
In addition, the digital signal receiving apparatus 70 receives the noise signal N by means of a second antenna 75. Then, the digital signal receiving apparatus 70 down-converts the received noise signal N to a desired frequency band at a high-frequency processing portion 77 of a reference noise signal-output portion 76. Further then, the digital signal receiving apparatus 70 generates a reference noise signal Nr, which is coherent with the noise signal N, by converting the signal N from analog to digital at an A/D converting portion 78, and outputs the digital signal to the noise-filtering unit 74.
The noise-filtering unit 74 generates a noise cancel signal AN via an adaptive filtering-portion 79 on the basis of the reference noise signal Nr, and filters the noise signal N by adding the digital signal (S″+N″) inputted from the antenna 71 to the noise cancel signal AN at an adder 80. A filter coefficient of the adaptive filtering portion 79 is sequentially updated by a filter coefficient-updating portion 81 so that the noise signal N (N″) is reduced to a minimum.
However, according to the digital signal receiving apparatus 70 of the reference 1, the second antenna 75 and the reference noise signal-output portion 76 (the high-frequency processing portion 77 and the A/D converting portion 78) are required for receiving the noise signal N, thus leads to an increase of a size of a circuit. In addition, certain ingenuity is required in order to prevent the desired signal S from being inputted to the second antenna 75. In other words, if the desired signal S is inputted to the second antenna 75, the desired signal S may be also generated as the noise cancel signal AN at the adaptive filtering portion 79 and the desired signal S may be filtered.
Then, another FSK digital signal receiving apparatus has been proposed in Japanese Patent No. 2005-45314A (hereinafter, referred to as reference 2). The FSK digital signal receiving apparatus according to the reference 2 includes an adaptive filter and filter controlling means. The adaptive filter includes an adaptive mode and a non-adaptive mode. In the adaptive mode, a filter coefficient is sequentially updated and an ambient noise is filtered at a signal-receiving standby state for receiving a desired signal. In the non-adaptive mode, an update of the filter coefficient is stopped at a signal-receiving state for receiving the desired signal. The filter controlling means controls switching to the adaptive mode and to the non-adaptive mode, of the adaptive filter. The adaptive filter includes an adaptive filter portion and a filter coefficient-updating portion. At the adaptive filter portion, the filter coefficient is changed in accordance with a change of the ambient noise at the signal-receiving standby state. The filter coefficient renewal portion sequentially renews the filter coefficient of the adaptive filter portion so as to reduce the ambient noise to a minimum. When a new signal is detected by detecting means at the signal-receiving standby state, the filter controlling means switches the mode of the adaptive filter from the adaptive mode to the non-adaptive mode and stops the update of the filter coefficient.
In such a case, at the signal-receiving standby state, the adaptive filter sequentially updates the filter coefficient and filters the ambient noise, reacting to the adaptive mode. On the other hand, at the signal-receiving state for receiving the desired signal, the adaptive filter stops renewing the filter coefficient reacting to the non-adaptive mode. In other words, at the non-adaptive mode, the adaptive filter filters the ambient noise utilizing the filtering coefficient updated for filtering the ambient noise at the adaptive mode. Accordingly, in the signal-receiving state for receiving the desired signal, even when the ambient signal and the desired signal have a correlation, the ambient signal is filtered by the adaptive filter and the desired signal is preferably received. In addition, even when the ambient signal and the desired signal have a correlation, the ambient signal and the desired signal can be selectively received because the filter controlling means controls the switching to the adaptive mode and the non-adaptive mode. Accordingly, the size of the circuit can be reduced in comparison with a configuration of the circuit in which the ambient signal and the desired signal are separately received.
However, according to the FSK digital signal receiving apparatus of the reference 2, when an interfering wave including a frequency being identical to that of the desired signal is combined, the desired signal may be approximately filtered because the filter coefficient is updated for filtering the ambient noise including the above-described frequency from the desired signal. Even when an electric power of the desired signal is larger than that of the interfering wave, the desired signal may be reduced to some extent by the adaptive filter and there is a possibility that the desired signal is not demodulated.
A need thus exists for an FSK digital signal receiving apparatus which can demodulate the desired signal more precisely.